Respiratory failure requiring pulmonary support affects in excess of 300,000 people in the United States per year. Approximately one-half of these patients suffer from adult respiratory distress syndrome (ARDS). Adult respiratory distress syndrome is an acute inflammatory lung disease with a mortality rate of 50%. This disease is characterized by increased capillary permeability resulting from the development of interstitial edema and alveolar flooding.
For the vast majority of patients with ARDS, there is no specific treatment, or supportive therapy. Supportive therapy for ARDS focuses on mechanical ventilation. Mechanical ventilation, by creating a positive pressure gradient, results in the inflation of the lungs. This is a reversal of the normal lung which functions by utilizing negative pressures to ventilate the lung.
Current ventilatory support may be damaging to the lung. Pulmonary "Volutrauma" secondary to high ventilator tidal volumes and airway pressures may cause a capillary leak syndrome pathologically indistinguishable from ARDS. Thus, alternative life support modalities such as extracorporeal membrane oxygenation (ECMO) may be a therapeutic option for acute respiratory failure in both infants and adults.
Oxygenators developed to date are broadly classified into bubble type and oxygenators and membrane type oxygenators. The membrane type oxygenators fall under the laminate type, the coil type, and the hollow fiber type. Membrane type oxygenators excel over the bubble type oxygenators since the membrane type oxygenators cause minimal blood damage, such as hemolysis, protein denaturation, and blood coagulation as compared with the bubble type oxygenators.
With the true efficacy of membrane oxygenators finally being realized, increasing success for the past decade has been realized in acute respiratory failure of the term gestation newborn. Encouraging results with extra corporeal membrane oxygenation (ECMO), has also been reported in the support of pediatric patients with life threatening pulmonary, or pulmonary vascular disease.
Despite the over 5,000 use of ECMO in the newborn by 1992, hardware innovation in this area has lagged far behind clinical advances. In fact, the ECMO circuit used today is remarkably similar to that reported over 20 years ago by Kolobow U.S. Pat. No. 3,969,240. As a result, progress in dealing with some of the more significant complications associated with ECMO support has been slow. These complications chiefly result from ECMO induced thrombocytopenia, the need for heparinization, and carotid artery ligation. The most commonly seen complications include intracranial bleeding, seizures, and post-ECMO functional deficits.
A typical pediatric ECMO circuit is composed of numerous components which include a venous reservoir, a roller (or impeller) pump, a membrane oxygenator, a heat exchanger, polyvinylchloride connecting tubing, and connectors. Blood is passively drained by gravity from the venous circulation using a siphon height of 100 cm or more into a collapsible bladder that acts as a compliant reservoir. The bladder has a proximity switch attached to its top surface that acts to regulate the roller pump by turning it off when the bladder deflates.
This mechanism limits the maximum suction applied to the patient to the hydrostatic pressure created by the siphon. Blood then passes through an occlusive roller pump and is forced at flow rates ranging from 120 to 170 ml/min/kg through a membrane oxygenator, such as a Kolobow U.S. Pat. No. 3,969,240. Oxygenated and CO.sub.2 cleared blood is then returned via a heat exchanger at body temperature back to the patient's circulation.
There are some obvious deficiencies associated with this type of system. These include: (1) a large area of contact between blood and potentially thrombogenic surfaces requiring substantial I.V. doses of heparin; (2) a relatively complex physical set-up which requires comparatively long runs of tubing subject to accidental kinking or catastrophic disconnection; and (3) a cumbersome and awkward method of regulating bypass flow rate, namely raising or lowering the whole apparatus to change the siphon pressure.
U.S. Pat. No 3,856,475 to Marx discloses a device for transferring oxygen into blood. Oxygen is dissolved into a fluorocarbon transfer medium. Oxygen depleted blood passes through a multiplicity of small diameter, blood gas pervious silicon membrane transporting tubes. The oxygen is transferred through the walls of the tube to the blood using direct gas infusion to the gas exchange medium, resulting in a non-homogeneous gas transfer medium secondary to bubble formation. Blood is propelled through the apparatus by compressing the gas exchange tubes.
U.S. Pat. No. 5,294,401 discloses a membrane type oxygenator for effecting exchange of gases with a porous gas exchange membrane possessing minute through pores forming a path for gas, which membrane type oxygenator is characterized by the fact that minute particles are retained in the minute pores of the porous gas exchange membrane to permit a decrease in the cross sectional area of the path for gas and as blood anticoagulant is retained in the minute particles or between the minute particles.
U.S. Pat. No. 5,277,176 discloses an Extracorporeal lung assistance apparatus and process which has an oxygen containing gas chamber positioned in such a relationship to a gas exchange chamber that transfer of oxygen to blood and the withdrawal of carbon dioxide from the blood. The above cited prior art, however, does not apply to neonate extracorporeal membrane oxygenators.
ECMO in neonates is also complicated by the low circulating blood volume of the patients and by the difficulty of obtaining vascular access. The total blood volume of a neonate is generally appreciably less than the priming volume of the typical ECMO circuit. A volume of donor blood equivalent to several total exchange transfusions is thus required simply to prime the circuit.
Therefore, a compact, simple to use pump/oxygenator system with reduced blood contacting surface area and automatic control would be an improvement in the treatment of acute respiratory insufficiency in both adult and neonatal patients. The present invention employs a two stage pump system in combination with a membrane oxygenator. The first pump is an axial flow pump whose primary function is to create secondary flows in the oxygenator at low pressure. Another function of the first pump is to provide controlled suction such that the hydrostatic suction, and venous reservoir required with conventional systems can be eliminated.
Since the laminar flow fluid boundary layer in membrane oxygenators is known to contribute the majority of diffusional resistance to gas transfer, the effect of induced secondary flows is to improve the membrane gas transfer efficiency, in this way, the membrane surface area required for adequate gas exchange may be reduced and the contact of blood with synthetic surfaces minimized.
The oxygenator portion of the invention consists of two readily made cylindrical membranes suspended in a housing to form a tubular flow channel. Blood passing through the oxygenator follows a complicated spiral path such that its effective contact time with the gas exchange membranes is prolonged.
The comparatively high fluid shear rates provided by the axial flow pump, in combination with smooth inflow and outflow transitions in the oxygenator, minimize the potential for thrombus formation. The second pump in the system is a servo-controlled centrifugal pump whose function is to return oxygenated blood back to the patient. It provides the energy needed to overcome the resistance of the return line and to maintain forward flow at either aortic or venous pressures, thus permitting the use of either veno-arterial or veno-venous bypass.
The control system of the new device employs fuzzy logic. The control system will automatically adjust the rotational rate of the centrifugal pump to maintain a low positive pressure in the oxygenator as the flow rate of the system is changed either in response to operator input or changing venous return flow, or to changes in the afterload pressure.
A compact and simple to use pump/oxygenator system with reduced blood contacting surface area and automatic control, combined with enhanced membrane transfer efficiency is a significant improvement in the treatment of acute respiratory insufficiency in adult and neonatal patients.